FAQ 5
Laser Safety and Free Electron Lasers
Based on the suggestions of large number of students and laser scientists, this section is being renamed as FAQ. The questions being addressed are of very general nature and are being forwarded to us by readers from different parts of India. This section provides an opportunity for interaction where one can ask their clarification either about the articles on web or any other question related to lasers. The reply will be sent to them after discussing with either the authors of a particular article or the experts in that field. Any comments / suggestions to improve the site welcome.
  1. How many types of Laser Hazards are there?
  2. What is the effect of various lasers on eye and skin?
  3. What is Maximum permissible exposure (MPE)?
  4. What is the MPE for various lasers?
  5. What is Nominal Hazard Zone?
  6. What are various Laser Classifications?
  7. How do you compare semiconductor lasers with solid-state lasers?
  8. What are the basic components of semiconductor lasers?
  9. What is the difference between homojunctions and heterojunctions in semiconductors?
  10. Draw a band energy diagram of p - n junction under no bias condition?
  11. Draw a band energy diagram of p - n junction under forward bias conditions? How Laser operation occurs in forward bias conditions?
  12. Define the Internal quantum efficiency of laser diodes?
  13. Why indirect band gap materials like silicon cannot be an efficient Laser devices?
  14. What are double-heterostructure (DH) lasers and what are the advantages?
  15. What is the difference between gain guided and index guided laser diodes?
  16. What are Fabry-Perot (FP) diode lasers?
  17. What are surface emitting laser diodes or Vertical Cavity Surface Emitting Laser (VCSEL)?
  18. What is Distributed Feedback Laser (DFB)?
  19. What are quantum well devices in Semiconductor Lasers?
  20. What are the most commonly used materials for Laser Diodes?
  21. What is typical output power vs. drive current characteristics?
  22. What is slope or Quantum efficiency?
  23. What are the typical characteristics of semiconductor lasers?
  24. What is the difference between an LED and a Diode Laser?
  25. What are the main applications of Diode Lasers?
  26. How Free electron lasers differ from other conventional lasers?
  27. What are the main subsystems of FEL?
  28. What is the resonant condition?
  29. How wavelength is changed in FEL?
  30. What are typical velocities of electrons in FEL?
  31. What are the typical parameters of FEL?
  32. How is linewidth related to undulator period?
  33. What are the main applications of FEL?


  1. How many types of Laser Hazards are there?
  2. Laser Beam Hazards, which include the damage to eye and damage to skin

    Non - Beam Hazards which include hazards from a number of support systems like high voltage, high current as well as radio frequency power supplies, high pressure arc and flash lamps, heavy duty capacitor banks, gases at high pressure in heavy containers, toxic gases and fumes, carcinogenic and inflammable materials, cryogenic systems etc. The non-beam hazards also include electrical hazards, explosion and fire hazards, and chemical hazards.


  3. What is the effect of various lasers on eye and skin?
  4. Biological Effects of lasers

    Wavelength Lasers Damage Remarks
    (200-280 nm) Argon Fluoride, Krypton chloride, Krypton Fluoride Eye Photokeratitis
    Skin Erythema (Sunburn), Skin Cancer
    (280-315 nm) Xenon chloride Eye Photokeratitis
    Skin Erythema (Sunburn), Accelerated Skin Aging, Increased Pigmentation
    (315-400 nm) Xenon Fluoride, Nitrogen, Helium Chloride Eye Cataract
    Skin Skin Burn, Pigment Darkening
    (400-780 nm) Helium Chloride, Helium Neon, Argon, Krypton, Copper Vapour, Frequency doubled Nd:YAG, Gold Vapour, Dye lasers (Visible), Ruby, Ti:Saphire(Visible), Diode laser (Visible) Eye Photochemical and Thermal Retinal Injury, Color and Night Vision Degradation
    Skin Skin Burn, Photosensitive Reactions
    (780-1400 nm) Ga As, Nd:YAG/Glass, chemical oxygen iodine laser (COIL) Eye Retinal Burns, Cataract
    Skin Skin Burn
    (>1400 nm) HF, Diode Lasers ( IR), He-Ne (IR), Erbium doped YAG/Glass, DF, Carbon Dioxide Eye Corneal Burn
    Skin Skin Burn



  5. What is Maximum permissible exposure (MPE)?
  6. MPE is defined in ANSI Z-136.1 as "the level of radiation to which a person may be exposed without hazardous effect or adverse biological changes in the eye or skin". The biological effects of laser radiation depend on the wavelength, exposure duration, repetition rate and power / energy levels. The MPE is usually expressed either in terms of radiant exposure in J/cm2 for pulsed lasers or as irradiance in W/cm2 for continuous lasers for a given wavelength and exposure duration. In general, the longer the wavelength, the higher the MPE and for longer exposure times, the MPE is lower.


  7. What is the MPE for various lasers?
  8. PULSED LASERS (MPE for 10 seconds duration - J/cm2)

    Laser Wavelength Pulse Width Eye Skin
    Argon Fluoride, Krypton chloride, Krypton Fluoride (200-280 nm) Nanosecond to tens of secs 3 x 10-3 3 x 10-3
    Xenon chloride (280-315 nm) Nanosecond to tens of secs 10-2 - 0.1 0.1
    Xenon Fluoride, Nitrogen, Helium Chloride (315-400 nm) Nanosecond to tens of secs 0.6 1.0
    Helium Chloride, Frequency doubled Nd:YAG, Gold Vapour, Dye lasers (Visible), Ruby, Ti:Saphire(Visible), Diode laser (Visible) (400-780 nm) Nanosecond to microsecond 5 x 10-7 2 x 10-2
    Ga As 905 nm Nanosecond to microsecond 1 x 10-6 1.5 x 10-2
    Nd:YAG 1064nm millisecond 5 x 10-5 1.0
    Nd:YAG 1064nm Nanosecond to microsecond 5 x 10-6 0.1
    Erbium doped YAG/Glass 1500nm nanosecond to millisecond 0.1 0.1


    CONTINUOUS LASERS. (MPE for 8-hour duration - W/cm2)

    Laser Wavelength Eye Skin
    Argon ion 488 / 514 nm 1 x 10-6 0.2
    Frequency doubled Nd:YAG 532nm 1 x 10-6 0.2
    He-Ne 632.8nm 1.7 x 10-5 0.2
    Nd:YAG 1064nm 1.6 x 10-3 0.2
    COIL 1.354 micron 4 x 10-2 0.2
    HF / DF 2.8 - 4.0 micron 0.1 0.1
    CO2 10.6 micron 0.1 0.1


  9. What is Nominal Hazard Zone?
  10. Nominal Hazard Zone (NHZ) is the area where the level of laser radiation is more than MPE and it is necessary to enforce various laser safety control measures to protect the users.


  11. What are various Laser Classifications?
  12. As per the American National Standards Institute, the ANSI Z136 laser safety standards classifies the Lasers in the following categories:
    • Class I lasers are considered safe, based upon current knowledge, under any exposure condition inherent in the design of the product. These Laser systems cannot emit laser radiation levels greater than the Maximum Permissible Exposure and are considered to be incapable of causing eye damage under normal operating or viewing conditions. Maximum power output is of the order of a few microwatts. These low powered devices that use lasers of this category include laser printers, CD players, and survey equipment, and they are not permitted to emit levels of optical radiation above the exposure limits for the eye. Lasers of this class are found in compact disc players. No safety requirements are specified for the use of this class of laser. It may be pointed out that the lasers which are totally enclosed system where access to higher levels of laser radiation is not possible during normal operation also falls in this category. However, whenever the instrument is opened for servicing or repairs, then these lasers are no longer fall in Class 1 and all the necessary precautions applicable to the embedded laser must be followed until the service is complete and the system is again enclosed.
    • Class 1M Lasers are considered incapable of producing hazardous exposure conditions during normal operations unless the beam is viewed through collecting optics like magnifying optical instruments such as telescope. These lasers produce either a large diameter beam or a highly divergent beam. Some of the lasers used for fibre-optic communication systems are Class 1M laser products. These lasers are exempted from any safety measures other than to prevent potentially hazardous optically aided viewing.
    • Class II is a low-power laser that usually emits in the visible portion of the spectrum (0.4 - 0.7 μm). The brightness of the beam normally causes the eyes to blink, well before any permanent damage can occur. These lasers are limited to a radiant power of less than 1 milliwatt, which is below the maximum permissible exposure for momentary exposure of 0.25 second or less. The natural aversion reaction to visible light of this brightness is expected to protect the eyes from damage, but any intentional viewing for extended periods greater than 1000 sec can result in damage. So deliberate staring into the beam should be avoided. Some examples of this class of laser are demonstration lasers for classroom use, laser pointers, laser printers and supermarket scanners.
    • Class II M class is similar to Class II in terms of power and wavelength. Normally these lasers produce either a large diameter beam or a highly divergent beam and as such the total output may be more as compared the output observed in Class II lasers. However, the power densities are safe for accidental viewing because of the diverging nature of laser beams. When viewed with the naked eye, the hazards are the same as for a Class II laser. But these lasers are potentially hazardous if viewed with collecting optics or certain optical aids like magnifying optical instruments e.g. binoculars or a telescope. Lasers used for surveying come under this Class.
    • Class III R lasers are continuous wave visible and infrared lasers with intermediate power levels of 1-5 milliwatts. These lasers have similar applications as encountered for Class II lasers, including laser scanners and pointers. They are considered safe for momentary viewing (less than 0.25 second i.e. blink response), but should not be viewed directly (intrabeam), or with any kind of magnifying optics. These medium-power laser systems though do not pose a fire hazard threat or even eye damage hazard through viewing of diffuse reflections, but nevertheless may be hazardous under direct and reflected beam viewing conditions. Lasers in this class may be used in alignment products.
    • Class III B lasers can be both in the visible and as well as infrared band and are of medium power: continuous wave (5-500 milliwatt), or pulsed (10 joules per square centimeter) These lasers are hazardous to the eye for direct intrabeam viewing and from specular reflections. In general these lasers are safe so far as diffuse reflections are concerned, except towards the high power end. However, longer wavelength and high powers can cause some skin damage.. Specific safety measures are recommended in the standards for control of hazards with this laser class. Examples of applications of this laser type are spectroscopy, confocal microscopy, and entertainment light shows. These lasers are also used in medical applications and research.
    • Class IV lasers emit high power, in excess of 500 mW, the limit for Class IIIB devices, and require stringent controls to eliminate hazards in their use. These lasers can be both continuous as well as pulsed in the visible and infrared ranges. These high-power laser systems are hazardous both to the eye as well as skin. Direct intrabeam viewing, specular and even diffuse reflection can cause severe eye and skin damage. These lasers can also have sufficient energy to ignite materials and thus are fire hazards. Also these lasers can produce hazardous plasma radiation, laser-generated air contaminants, hazardous fumes and byproduct emissions as a result of laser matter interaction. Since most laser eye injuries involve reflections of Class IV laser light, their use requires extreme caution. All reflective surfaces must be kept away from the beam, and appropriate eye protection worn at all times when working with these lasers. In case of pulsed lasers of this class, the power supplies can be fatal and thus all electrical safety precautions must be taken. Class IV lasers can be found in the metal industry, research laboratories, and laser light shows. These lasers are employed for surgery, cutting, drilling, micromachining, and welding.

  13. How do you compare semiconductor lasers with solid-state lasers?
  14. Solid-state and gas lasers work on narrow optical transitions connecting discrete energy levels between which population inversion is achieved by optical or electrical pumping


    Semiconductor lasers, on the other hand, work on transitions between energy bands in which conduction electrons and valence holes radiative recombination across the band gap that determines the emission wavelength.


    As compared to other lasers, semiconductor lasers are:
    • Compact and rugged. This ruggedness and small size allow laser diodes to be used in environments and spaces in which other types of lasers cannot operate
    • High efficiency in the range of 30 - 80%
    • Direct excitation with small electric currents,
    • Possibility of direct modulation with applied current
    • Small beam waist
    However, there are few drawbacks in semiconductor laser diodes as compared to other solid state and gas lasers. These include, their sensitivity to temperature and large beam divergence.


  15. What are the basic components of semiconductor lasers?
  16. Semiconductor lasers also has basic three components
    • A gain medium that amplifies light (p-n junction)
    • An energy source to create population inversion (electrical current through the junction)
    • A resonant cavity for confining the light (cleaving the semiconductor creates a reflective facet which can be used to create a laser cavity). Further to reduce the threshold for laser operation, the reflectivity of output coupler and the total reflector can be realized using dielectric coatings.


  17. What is the difference between homojunctions and heterojunctions in semiconductors?
  18. An interface between two regions of a semiconductor or an interface between two different semiconductor materials is called a junction. Junctions between differently doped regions of the same semiconductor material are called a homojunction, while a junction between two different types of materials is called a heterojunction. A junction between a p-type and an n-type semiconductor is called a p-n junction.


  19. Draw a band energy diagram of p - n junction under no bias condition?
  20. Simplified p-n junction diagram under no bias When the contact is made between the 'n' and the 'p' doped material, electrons diffuse from the n region into the p region where they recombine with the abundant holes. Similarly holes diffuse from p region to n region and combine. Electrons leave behind the positively charged donor ions, so some part of n region will be positively charged. Similarly some part of the p side will be negatively charged. Due to the diffusion of both types of carriers away from the junction region, a narrow zone around the junction is totally depleted of mobile charge carriers. This region is called the depletion region. The process happens till dynamic equilibrium takes place: the diffusion of electrons/holes and the drift currents cancel, so in the absence of an external field no net current flows across the junction. In terms of band structure, p-n junction can be represented as shown in the figure:

    Note that the systemcw of a p-n junction without bias is in equilibrium and hence the Fermi level EFN for n-type and EFP for p-type must be equal implying that there will be band bending. Thus in the absence of a bias, the bottom of the conduction band on the n-side lies lower than that on the p-side. This prevents net diffusion, as the electrons have to overcome a potential barrier qφ.


  21. Draw a band energy diagram of p - n junction under forward bias conditions? How Laser operation occurs in forward bias conditions?
  22. Positive voltage to the p region and negative voltage to the n region is known as forward bias. This allows the current to flow through the junction.

    On the other hand, the junction is reverse biased if a negative voltage is applied to the p region and positive voltage is applied to n region. Under reverse biased condition, very little current small current flows.

    Under forward bias conditions, if the external voltage becomes greater than the value of the potential barrier, the current will start flowing through the junction. This is because the negative voltage pushes electrons towards the junction giving them the energy to cross over and combine with the holes, which are being pushed in the opposite direction towards the junction by the positive voltage. Thus forward bias creates extra charge carriers in the junction, lowers the potential barrier, and causes injection of charge carriers, through the junction, to the other side.

    Simplified p-n junction diagram under no bias The laser operation occurs at a p-n junction, that is the boundary region between p-type and n-type materials. When p-n junction diode is forward biased, then there will be injection of electrons into the conduction band along n-side and production of more holes in valence band along p-side of the junction. At the junction, electrons and holes meet and are attracted to each other because of opposite charges. When they meet, they recombine and emit radiation. When a forward-bias voltage is applied to the junction, the barrier height is reduced and some of the electrons in the conduction band will overlap some of the holes in the valence band. It is worth pointing out that pumping the semiconductor raises some electrons to the conduction band where they rapidly distribute themselves into the lowest available energy levels within the conduction band. On the other hand, the electrons in the valence band occupy the lowest energy levels there, pushing the holes to the top of the valence band. Thus under forward bias conditions, there are more number of electrons than the number of holes in the junction region because of higher mobility of electrons as compared to that of holes. In other words a population inversion, the necessary condition for laser operation. In this situation, radiative recombination of the holes and electrons can occur. Electrons fall across the energy gap and recombine with holes. At very low currents, a population inversion does not occur even though recombination radiation is emitted. Under these conditions p-n juction behaves as a light-emitting diode (LED). In comparison, to produce a population inversion, comparatively high current is required within the junction region. This situation is indicated in the adjoining figure where p-n junction is shown under forward bias conditions.


  23. Define the Internal quantum efficiency of laser diodes?
  24. The efficiency for an efficient device can be characterized by the term "internal quantum efficiency" ηINT, defined as
    ηINT   =   Number of band to band radiative recombinations

    Number of carriers crossing junction
    and is given as
    Internal quantum efficiency
    where A is an Einstein coefficient describing radiative recombination and X is a coefficient for nonradiative recombination. Thus it is required that A >> X for efficient photon generation.


  25. Why indirect band gap materials like silicon cannot be an efficient Laser devices?
  26. Indirect-gap semiconductors are inefficient light emitters because in case of indirect band gap materials like silicon, transitions between conduction bands to valence band involve phonon for conservation of momentum. Moreover phonon-assisted photon emission involves three "particles" simultaneously (electron, photon and phonon), its probability is low. Further,
    Internal quantum efficiency
    where A is an Einstein coefficient describing radiative recombination and X is a coefficient for nonradiative recombination. Thus it is required that A >> X for efficient photon generation. In case of indirect band gap materials, the value of X is very high.


  27. What are double-heterostructure (DH) lasers and what are the advantages?
  28. The double-heterostructure (DH) laser diode, which consists of a thin layer of low bandgap material such as GaAs sandwiched between two high bandgap layers such as AlGaAs, is one of the most commonly studied geometry. The bandgap discontinuity confines the free electrons and holes to the active region, meaning that more electron-hole pairs can contribute to the amplification. Further, the semiconductor with a wider band gap (AlGaAs) will also have a lower refractive index than GaAs. This difference in refractive index is what establishes an optical dielectric wave-guide that ultimately confines photons to the active region. Use of such structures help in confining both the injected electrons and holes and also the emitted photons to a narrow region about the junction. This as such requires less current to establish the required concentration of electrons for population inversion. Typically, the DH laser has a room temperature threshold current density two orders of magnitude smaller then the homojunction device.


  29. What is the difference between gain guided and index guided laser diodes?
  30. This confinement of the laser operation within a stripe region is usually accomplished by either gain guiding or by index guiding. Both these methods confine the light in such a way that the losses due to beam spreading are minimized thereby reducing the current requirements for laser operation. Further, since in the stripe geometry, aperture is limited and the dimensions in the directions parallel and perpendicular to the junction are comparable thus reducing astigmatism. Index-guided lasers employ steps in the index of refraction both parallel and perpendicular to the junction to confine the light. On the other hand, gain guiding structure makes use of composition changes for confinement in the plane of the junction. This is done, by adjusting the charge carrier density in the region, which results in the required refractive index. However, refractive index changes in the direction perpendicular to the junction to confine the light, just as like index-guided devices.


  31. What are Fabry-Perot (FP) diode lasers?
  32. Most of the laser diodes are edge emitters that are also called Fabry-Perot (FP) diode lasers since the cavity is essentially similar to that of a conventional gas or solid-state laser but formed inside the semiconductor laser diode chip itself. The mirrors are either formed by the cleaved edges of the chip or one or both of these are anti-reflection (AR) coated and external mirrors are added for high performance.


  33. What are surface emitting laser diodes or Vertical Cavity Surface Emitting Laser (VCSEL)?
  34. Surface emitting laser diodes (VCSEL: Vertical Cavity Surface Emitting Laser) have also become of interest for special applications. VCSEL have the optical cavity axis along the direction of current flow rather than perpendicular to the current flow as in conventional laser diodes. The active region length is very short compared with the lateral dimensions so that the radiation emerges from the surface of the cavity rather than from its edge. VCSELs emit their beam from their top surface. This approach provides several very significant technical advantages in terms of Beam characteristics and lasing threshold. The beam from a typical VCSEL exits from a circular region 5 to 25 um in diameter. Since this is much larger than for the FP laser diode, the divergence of the resulting beam is much lower. And, because it is also circular, no corrections for asymmetry and astigmatism are required - a simple lens should be able to provide excellent collimation. Lasing threshold drive current is an order less than the edge emitting laser diodes. Further, the packing density of such devices can be an order of magnitude higher than for FP laser diodes. As of now the output power is less and VCSEL technology is in its infancy its potential is just beginning to be exploited.


  35. What is Distributed Feedback Laser (DFB)?
  36. Distributed Feedback Lasers (DFB) is a special category of lasers under edge emitters, which incorporates a distributed grating that acts as a distributed reflector. This results in single mode lasers of high stability, which is the requirement of telecom industry.


  37. What are quantum well devices in Semiconductor Lasers?
  38. Recently quantum well devices for semiconductor technology are being pursued seriously. A quantum well is a very thin layer of semiconductor material between two layers with larger values of band-gap. If the layer is thin enough, 20 nm or less, comparable to the deBroglie wavelength, (λ ≈ h/p), quantum mechanical properties of electrons become important. This changes the energy-level structure of the material. Quantum well devices may incorporate a single quantum well or multiple quantum wells, with a number of alternating thin layers of high-band-gap and low band-gap material. The use of quantum wells in laser devices allows optimizing the properties of the material for the specific application. Quantum well devices offer lower threshold current and higher output power than devices without quantum wells.


  39. What are the most commonly used materials for Laser Diodes?
  40. Most commonly used material for semiconductor lasers are the III-V compounds such as GaAs, AlGaAs, InGaAs and InGaAsP depending upon the desired lasing wavelength. Recently, GaN/AlGaN and InGaN/AlGaN are also being used to achieve emission in the blue and ultraviolet regions.


  41. What is typical output power vs. drive current characteristics?
  42. Figure shows the output power of a semiconductor lasers as a function of current. Above a threshold current, at which the laser diode starts lasing, the laser diode shows almost a linear dependence between optical output power and laser current. Below the threshold the spontaneous emission is predominant and the optical amplification is not sufficient the device behaves like a LED.

    Diode output power vs drive current


  43. What is slope or Quantum efficiency?
  44. Refer to above figure. In the linear region, the slope of the output vs current curve yields the electrical-to-optical power conversion efficiency, also known as slope or quantum efficiency. The values of slope efficiency vary from 30 - 80 %.
    Quantum efficiency


  45. What are the typical characteristics of semiconductor lasers?
  46. The output characteristics of these devices are slightly different from those of other type of lasers. Because of their small size these have beam divergence angles of as much as high as 20o - 30o. The high value of divergence of semiconductor lasers is because of diffraction of the light waves when couple out of the laser structure

    Since the active light-emitting area is rectangle-shaped with different length and breadth, the parallel and vertical divergence are also different. If we focus such a beam, it will be observed that the focus of the vertical and the focus of the parallel divergence are not congruent but are shifted against each other: the effect known as astigmatism.

    The characteristic curve (output power vs. current) of a semiconductor laser strongly depends on the temperature. Higher the temperature, higher is the threshold current and smaller is the slope of the curve in the laser region.

    The coherence length of semiconductor laser diodes is low. Typical values for an index guided Fabry-Perot laser, emitting a single spectral line at 825 nm is 7cm, whereas for a gain guided Fabry-Perot laser, the coherence length is 300μm only.


  47. What is the difference between an LED and a Diode Laser?
  48. Difference between Diode Laser can be summarized as follows:

    Semiconductor Laser LED
    Generation through stimulated emission Generation by spontaneous emission
    Monochromatic and coherent light beam Divergent and incoherent light beam
    Power output kilowatts Power output in miliwatts
    Require feedback mechanism like optical resonator Does not require feedback mechanism
    Expensive Cheap
    Requires temperature and current stability Easy to handle. No such controls are required
    Generally spectral width less than 5 nm Spectral width upto 100 nm


  49. What are the main applications of Diode Lasers?
  50. Main applications of Diode lasers are in the following areas:
    • Telecommunication
    • Optical storage
    • Solid state Laser pumping
    • Material processing such as Welding, drilling and cutting
    • Medical applications in dermatology, dentistry, ophthalmology, in surgery of tumors, kidney stone
    • Barcode scanning
    • Inspection, measurement and control
    • Laser printers
    • DVD drives
    • Laser pointers

  51. How Free electron lasers differ from other conventional lasers?
  52. Radiation from a Free Electron Laser (FEL) has many common features in common with radiation from a conventional optical laser, such as high power; narrow bandwidth and diffraction limited beam propagation. One of the main differences between the two lasers is the gain medium: In a conventional LASER, the amplification comes from the stimulated emission of electrons bound to atoms, either in a crystal, liquid dye or a gas, whereas the amplification medium of the FEL are "free" (unbound) electrons. The free electrons have been stripped from atoms in an electron gun and are then accelerated to relativistic velocities.


    Ordinary lasers, however, operate at a fixed frequency. Though efforts have been made to have large number of wavelengths by tuning or having second or third harmonic generation, but still the choice is limited. That limits their usefulness. However, the FELs are ideal for exploring the unknown regions in the spectrum because these are tunable over a broad range of the spectrum. That enables these lasers more useful for material, medical and military applications.


  53. What are the main subsystems of FEL?
  54. Basic scheme of a free electron laser The basic FEL system consists of an electron accelerator, an undulator or wiggler in which the electrons emit the syncrotron radiation, and an optical resonator. In FEL, a beam of electrons is accelerated to almost the speed of light. The beam passes through the FEL oscillator, a periodic transverse magnetic field produced by an arrangement of magnets with alternating poles within an optical cavity along the beam path. This array of magnets is called an undulator, or a wiggler, because it forces the electrons in the beam to follow a sinusoidal path. The acceleration of the electrons along this path results in the release of photons (synchrotron radiation). Since the electron motion is in phase with the field of the light already emitted, the fields add together coherently resulting in an exchange of electron energy with the electromagnetic field. This process is induced by the interaction of the electromagnetic radiation with the electrons. Since the radiation is faster than the electrons speeding along their path, the radiation overtakes the electrons flying ahead and interacts with them along the way, accelerating some of them and slowing others down. As a result of energy exchange, the electrons that gain energy begin to move ahead of the average electron, while the electrons that lose energy begin to fall behind the average. In the process, the beam of electrons gradually gets bunched on the scale of the radiation wavelength and this collective motion of bunches radiates powerful coherent synchrotron radiation.


  55. What is the resonant condition?
  56. Over one undulator period, λw, the time difference between the electron bunch and the wave must correspond to the wavelength, λo, of the spontaneously emitted light. Under resonance condition, the wavelength of the emitted radiation, λo, at the resonance depends on the electron energy and the magnitude and periodicity of the undulator and the magnetic field strength according to the relation
    Undulator
    γ is the relativistic factor and γmc2 is the energy of electrons. K is the undulator parameter, which is proportional to the magnetic field inside the undulator and is given as
    Undulator
    Where Bw is the undulator magnetic field strength in Tesla and λw is the undulator period length in centimeters.


  57. How wavelength is changed in FEL?
  58. The wavelength of the light emitted can be readily tuned by adjusting the energy of the electron beam or the magnetic field strength of the undulators.


  59. What are typical velocities of electrons in FEL?
  60. One can have an idea about the velocity of electrons from the relation:
    Velocity of electrons
    Where ν is the velocity of electrons, c is the velocity of light and γ is a factor related to energy of electrons as:
    Energy of electrons
    The electrons having energy of one MeV will have velocity of about 86 % of the velocity of light. Similarly, electrons with energy of 10 Mev and 100 MeV will have velocity of the order of 99.9 % and 99.999% of the velocity of light.


  61. What are the typical parameters of FEL?
  62. Typical values of various parameters are given below:
    • Peak Magnetic field: few kilogauss
    • Wavelength: few Angstroms to 100 mm
    • Number of undulator periods: 100
    • Undulator period λw: 2 - 10 cm
    • Length of Undulator: 10 meters
    • Electron beam energy: Few MeV to Several GeV
    • Electron beam radius: About 1mm
    • Electron beam pulse: nanoseconds to femtoseconds
    • Efficiency: up to 40 % at longer wavelengths but less at shorter wavelengths
    • Photon beam divergence (FWHM) < μrad
    • Pulse duration (FWHM) ~ 100 fs
    • Excellent beam quality M2 < 1.1


  63. How is linewidth related to undulator period?
  64. The linewidth of the laser radiation is determined by the number Nw of undulator periods; larger the number of periods, narrower is the line width. The linewidth is given as
    Δλ / λ = 1 / 2 Nw

  65. What are the main applications of FEL?
    • Material science for micro machining, metal surface processing, polymer surface processing, electronic material processing, Nanotube synthesis
    • Atmospheric research,
    • Isotope separation
    • Spectroscopic tools for imaging, and to probe dynamical processes in real time on timescales down to tens of femtoseconds.
    • The development of X-ray Free electro lasers will help researchers to take snapshots of chemical bonds being made and broken, and to look at detailed physical processes such as planes of atoms sliding over one another.
    • Medical applications like surgery where the beam needs enough energy to vaporize soft tissue and bone. Some of these applications may be based on the clean cutting of soft tissue. Other uses may include welding tissue to assist in wound healing, repairing nerves, reattaching retinas or monitoring neurological activity. Wavelengths particularly near 6.45 microns have been found optimal for cutting all soft tissues. On the other hand, two wavelengths 7.5 and 7.7 microns have been found to cut through bone particularly cleanly.
    • The progress in Free electron lasers can help in developing dynamic imaging techniques for diagnosing conditions such as progressive degenerative diseases and cancer.
    • Though Army and Air force are concentrating on Solid State lasers and Chemical Lasers respectively, Navy however is pursuing mainly Free Electron Lasers. As per the reported data, Free electron lasers capable of delivering 14 kW have already been developed with immediate goals of developing 100 kW and upgrading later on to a Megawatt level. Magazine depth is an important issue The FEL magazine is much deeper as compared to that of other lasers.


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Updated: 6 April, 2015
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